Heat oven to 350°F. Grease, lightly flour and line the bottom of two 9-inch round cake pans with parchment paper.
In a medium-size bowl, whisk together the flour, cornstarch, salt, baking powder and baking soda. Set aside.
In the bowl of a standing mixer or in a large bowl with a hand-held mixer, cream together the butter and granulated sugar on high speed until fluffy, about 3 minutes, scraping the side of the bowl as necessary.
Reduce the mixer to low speed. Add the whole eggs and the egg white, one at a time, to the creamed butter mixture.
Add the vanilla and buttermilk; mix until well combined.
In 3 separate additions, add the dry ingredients to the wet, mixing after each just until combined. Fold in the chocolate chips.
Pour the cake batter evenly between the two cake pans; place in the oven.
Bake for 22 to 25 min. or until you insert a toothpick or a cake tester into center of cake, and it comes out clean.
Cool the cakes in the pans on wire racks until completely cool.
Add the powdered sugar, cocoa powder, PHILADELPHIA Cream Cheese, butter, and vanilla to the bowl of a standing mixer or to a large bowl with a hand-held mixer. Mix together, increasing the speed of the mixer gradually.
Whip the frosting ingredients together until fluffy, about 2 min. Gradually add the whole milk, whipping after each addition. Keep adding the milk until you reach the desired consistency.
Place the first cooled cake layer on a cake stand or plate.* Spread a layer of frosting on top of the first layer, and then place the second layer on top of the first.
For a smooth cake, apply a crumb coat of frosting. To apply the crumb coat, apply a thin amount of frosting all over the cake. The cake should still show through and crumbs should be sticking to the frosting. Refrigerate the cake for 20 min. This seals the crumbs to the cake so the final layer of frosting goes on evenly.
After the crumb coated cake has chilled, apply the rest of the frosting and decorate as desired.
For easy slicing, chill the cake an hour before serving.